1. Field of the Invention
The present invention relates to the field of fast settling amplifiers.
2. Prior Art
In current-sense applications for power management of cell phones, laptops, or electric cars, a small differential-mode (DM) voltage (in the order of mVolts) has to be measured across a current-sense resistor, in the presence of a large common-mode (CM) voltage on the sense resistor on the order of Volts or tens of Volts higher than the amplifier supply voltage. Instrumentation amplifiers (IAs) are frequently used for sensor interfacing. To solve the DM voltage measurement while the CM voltage is beyond the supply-rail, often a part of the input circuitry such as a resistor bridge or a voltage-to-current converter is allowed to draw its CM supply current from the sense resistor. This input CM supply current often has an undesired influence on the system to be measured. Therefore, IAs with capacitive-coupled input choppers have been developed that do not draw input CM supply current, while able to sense beyond their supply-rail voltage.
Capacitive-Coupled Amplifiers with Slow Settling
The prior art capacitive-coupled operational amplifiers and instrumentation amplifiers from the book “Operational Amplifiers, Theory and Design, Second Edition” (2011) of Johan Huijsing, one of the present inventors, do not have a straight frequency characteristic at the chopping frequency. These amplifiers have a slow-settling ripple at the chopping frequency after a step in the input signal. Therefore these amplifiers are not fit for fast-settling signal transfer. Their useful bandwidth lies below the chopper frequency.
The chopper-stabilized amplifiers of FIGS. 1 and 2 have an RC low-pass filter between the input chopper and the chopper behind, which acts as a notch filter in the signal transfer. This means that there is no feedback at the chopping frequency. Therefore an uncontrolled up-turn of the chopper ripple can occur. This causes a slow-settling ripple component after a step.
The chopper amplifiers of FIGS. 3, 4 and 5 have a ripple reduction loop. This means by definition, there is a notch in the frequency response at the chopper frequency. That also means there is a slow-settling ripple at the clock frequency after a step.